Carbon fiber or C—C composites which are useful for instance in airplane braking systems are subject to oxidation and resultant weight loss (that is, loss of mass). Oxidative weight loss of such carbon composites is generally retarded by coating articles made of the carbon composites with an antioxidant coating.
U.S. Pat. No. 6,737,120 B1 relates to carbon fiber or C—C composites that are useful in a variety of applications. This patent teaches methods of protecting such composites against oxidation by coating them with fluidized-glass type mixtures. The fluidized-glass mixtures are maintained as liquid precursors and are applied to components formed of carbon fiber or C—C composites. Once coated with the precursors, the coated C—C components are heat-treated or annealed for one or more cycles through a series of gradual heating and cooling steps. This creates glass coatings having thicknesses of about 1–10 mils. The thicknesses of the glass coatings may be varied by varying the composition of the fluidized glass precursor mixtures, the number of application cycles, and/or the annealing parameters.
U.S. Pat. No. 6,737,120 teaches that the fluidized glass materials may comprise such materials as borate glasses (boron oxides), phosphate glasses (phosphorus oxides), silicate glasses (silicon oxides), and plumbate glasses (lead oxides). These glasses may include phosphates of manganese, nickel, vanadium, aluminum, and zinc, and/or alkaline and alkaline earth metals such as lithium, sodium, potassium, rubidium, magnesium, and calcium and their oxides, and elemental boron and/or boron compounds such as BN, B4C, B2O3, and H3BO3. By way of example, U.S. Pat. No. 6,737,120 discloses a boron-containing liquid fluidized glass precursor mixture that includes 29 weight-% phosphoric acid, 2 weight-% manganese phosphate, 3 weight-% potassium hydroxide, 1 weight-% boron nitride, 10 weight-% boron, and 55 weight-% water.
U.S. Pat. No. 6,455,159 B1 likewise relates to carbon-carbon composites and graphitic materials. U.S. Pat. No. 6,455,159 has as objectives the protection of carbon/carbon composites or graphites at elevated temperatures up to and exceeding 850° C. and the reduction of catalytic oxidation at normal operating temperatures. U.S. Pat. No. 6,455,159 achieves these objectives by employing a penetrant salt solution which contains ions formed from 10–80 wt-% H2O, 20–70 wt-% H3PO4, 0.1–25 wt-% alkali metal mono-, di-, or tri-basic phosphate, and up to 2 wt-% B2O3. Their penetrant salt solutions also include at least one of MnHPO4.1.6H2O, AlPO4, and Zn3(PO4)2, in weight-percentages up to 25 wt-%, 30 wt-%, and 10 wt-%, respectively.
Application Ser. No. 10/223,946, filed Aug. 20, 2002 (H0003342), discloses a coated article protected against catalytic oxidation when the article is subjected to temperatures of 800° C. or greater. The article comprises a component made of carbon fiber or carbon-carbon composite annealed at a temperature in the range of 1600–2600° C. This component is covered by a phosphorus-containing undercoating comprising ions formed from 10–80 wt-% H2O, 20–70 wt-% H3PO4, 0.1–25 wt-% alkali metal mono-, di-, or tri-basic phosphate, 0–2 wt-% B2O3, and 0–25 wt-% MnHPO4.1.6H2O, 0–30 wt-% AlPO4, and 0–10 wt-% Zn3(PO4)2, provided that at least one of AlPO4, MnHPO4.1.6H2O, and Zn3(PO4)2 is present. The undercoating is covered by a boron-containing glass overcoating. The coating systems of Ser. No. 10/223,946 provide a high level of oxidation protection simultaneously in both high temperature and catalyzed oxidation conditions. However, these coating systems may become somewhat “muddy” in appearance after long-term humidity exposure. This may make them unsatisfactory for field use.
Silicate binder compositions have long been well known. See for instance U.S. Pat. Nos. 4,504,314; 4,391,642; 4,329,177 and 2,995,453. However, silicate binders have not previously been used in the context in which they are employed in the present invention.
The coating systems described above provide significant antioxidant protection for carbon composites. However, there remains a need for antioxidant coating systems that provide improved humidity resistance.